1. Field of the Invention
The present invention relates generally to the generation of blanking pulses in a cathode-ray tube and, more particularly, to a blanking pulse generating circuit adapted for use in a television receiver where the horizontal scanning frequency is selectively changeable.
2. Description of the Background
In a television receiver, the coil for deflecting the electron beam in the cathode ray tube (CRT) is supplied with a sawtooth signal whose waveform inclination or slope varies in a tracing interval and a flyback interval, and in which the electron beam is interrupted during each flyback interval to improve the image quality.
It has been customary that a blanking pulse applied to interrupt the electron beam during the flyback interval is formed by slicing. An example of a so-called slicing circuit is shown in FIG. 5, in which a flyback pulse P.sub.b is output from a horizontal flyback transformer FT that is coupled to a horizontal output transistor Q and a damper diode D, which form a deflection circuit for supplying a sawtooth waveform signal to a deflection coil DY adjacent the CRT (not shown). Nevertheless, because it is impossible in this case to execute blanking by a predetermined amount in advance of a synchronizing signal, there occasionally appears a folded incomplete image at the right-hand side of the CRT screen, particularly in a television receiver where overscan is not performed. Consequently, a complicated waveform processing circuit is typically required in order to obtain the proper blanking pulse.
Furthermore, there is another problem such that if the blanking interval is prolonged by a waveform shaping circuit, the left-hand side of the image is rendered defective on the CRT screen of the monitor.
More particularly, in a television receiver that is designed for monitoring video signals of different horizontal scanning frequencies, such as in the case of a receiver intended for receiving both high definition television (HDTV) broadcasts and ordinary television broadcasts, the relative phase and duration of the blanking signal are changed upon the input of video signals of such different television systems, which consequently results in deteriorating the quality of the monitor image. Meanwhile, the monitor receiver is usually used in most broadcasting stations for displaying and watching a synchronized image portion at the center of the CRT screen. Thus, some difficulties are presented in order to attain a proper blanking interval in such operation.